Exploring the origin of PLC bands in Hastelloy using in situ thermo-mechanical experiments

• Method for simultaneous full-field temperature and strain mapping above 1000 °C. • Sub-micron optical resolution achieved beyond 1000 °C. • Optical DIC extended beyond 1000 °C using dual filtering and reference image methods. • A, B, and C-type serrations in Hastelloy do not reflect local material behavior . • PLC band mechanisms involve stress drops, elastic recovery , localized hardening/softening, and slip dynamics. Experimental determination of the mechanical behavior at high temperatures (>1000 °C) has been a challenge both in terms of creating the high-temperature environment, and measurement of thermal and deformation fields, accurately. This work proposes a novel method for high resolution and simultaneous, full-field temperature and deformation measurements, from a single surface, at very high temperatures using direct and localized specimen heating. Visible light two-dimensional digital image correlation was employed to compute in-plane displacements. All camera sensor planes were kept parallel to the sample surface to minimize out-of-plane displacement corrections and reduce the need for a large depth of field, ensuring one-to-one mapping of thermal and strain fields. The saturation of the unfiltered white light, which generally occurs above 500 °C, was avoided by separating the visible light from infrared spectral bands while narrowing it to the blue region. In situ tensile experiments on Hastelloy-X were conducted to demonstrate the benefits of the novel experimental methods. The results showed the capability of the heating system to maintain a uniform temperature in the predefined ROI, which remained stable over time. The dual filtering and selection of reference image was shown to be effective in extending the utility of visible light DIC beyond 1000 °C. The high-resolution data from tensile experiments on Hastelloy-X revealed that the conventional "A", "B", and "C" type serrations in the stress-strain curves, in the temperature range of 300–700 °C were characteristics of global load-displacement curves due to lower strain resolution. Locally, the mechanical behavior was governed by sudden stress drops due to PLC bands, elastic recovery, and plastic flow with local hardening or softening, differing from global hardening response.